1. Field of the Invention
The present invention relates to a compatible optical pickup, and, more particularly, to a compatible optical pickup designed to record and/or reproduce information on and/or from recording media having different formats such as high density recording media using blue-violet light and lower density DVD and/or CD formats.
2. Description of the Related Art
In an optical recording and/or reproducing apparatus for recording and/or reproducing information to and/or from a disc type recording medium using a light spot focused by an objective lens, the recording density is determined by the size of the light spot. Equation (1) defines a light spot size S that is determined by a wavelength λ of light and a Numerical Aperture (NA) of the objective lens:S∝λ/NA  (1)
Since the emergence of a CD technology designed to record and/or reproduce information using light having a wavelength of 780 nm and an objective lens with an NA of 0.45 or 0.5, much research has been conducted to increase data storage capacity by increasing the areal recording density. This research has resulted in a DVD technology designed to record and/or reproduce information using light having a wavelength of 650 nm and an objective lens with an NA of 0.6 or 0.65.
Ongoing studies are now underway to develop high density recording media that can offer over 20 GB of recording capacity using blue-violet light of a predetermined wavelength, e.g., 405 nm. Standards on high density recording media are actively under development, some of which are almost finalized. The standards specify use of blue-violet light of a 405 nm wavelength and an objective lens with an NA of 0.65 or 0.85, which will be described below.
The standards also set a thickness of a DVD disc to 0.6 mm, which is 50% less than the 1.2 mm of a conventional CD disc. This is aimed at providing a tolerance due to a tilt of the recording medium, since the NA of a DVD disc's objective lens increases to 0.6, which is higher than the 0.45 NA of a conventional CD disc's objective lens.
In a case in which a tilt angle of a recording medium is θ, the refractive index is n, its thickness is d, and the NA of an objective lens is NA, a coma aberration W31 caused by a tilt of the recording medium is defined by Equation (2):
                              W          31                =                              -                          d              2                                ⁢                                                                      n                  2                                ⁡                                  (                                                            n                      2                                        -                    1                                    )                                            ⁢                                                          ⁢              sin              ⁢                                                          ⁢              θ              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                      (                                                      n                    2                                    -                                                            sin                      2                                        ⁢                    θ                                                  )                                            5                /                2                                              ⁢                      NA            3                                              (        2        )            Here, the refractive index and thickness of the recording medium refers to those of an optical medium from an incident surface to a recording surface.
As is evident by Equation (2), in order to provide a tolerance due to the tilt of the recording medium, the thickness of the recording medium must be reduced if the NA of the objective lens is increased for high-density recording.
Thus, for example, if the NA of the objective lens for a high density recording medium is increased to 0.85, the high density recording medium must be about 0.1 mm thick. A blu-ray disc (BD) is a recording technology that increases the recording density by increasing the NA of an objective lens while reducing the thickness of the recording medium. A BD standard specifies a 405 nm wavelength light source, 0.85 NA objective lens, and an approximately 0.1 mm thick recording medium.
Here, examples of the high-density recording medium include a BD and an advanced optical disc (AOD). The AOD uses a light source with a 405 nm wavelength and an objective lens with a 0.65 NA. The thickness of the recording medium is 0.6 mm, as in the DVD. While the BD achieves a higher recording density than the AOD, the AOD is advantageous over the BD because of the AOD's reverse compatibility with the DVD format and the easy manufacturing of the AOD due to the NA of the objective lens and the disc thickness being the same as those of the DVD. Meanwhile, for write-once media such as DVD-R and CD-R, light sources with 650 nm and 780 nm wavelengths must be used since reflectivity significantly decreases according to wavelength.
Thus, an optical pickup designed for compatibility between the high density recording media using blue-violet light and the lower density DVD and/or CD formats uses two objective lenses for the high density recording media and DVD and/or CD, taking into consideration a spherical aberration caused by a thickness difference between the two types of recording media formats and the NA required for each recording media format. In addition, considering compatibility with DVD-R and/or CD-R formats, the optical pickup for high density recording media adopts two or three light sources having different wavelengths. In this case, optical systems for high density recording media and DVD and/or CD discs have different configurations.
Thus, due to the complicated optical system and large number of optical components, the conventional compatible optical pickup has several problems such as high manufacturing costs, low reliability, and degradation in assembling and adjustment capabilities. The low reliability is caused by a deviation between the optical components during high temperature operation. Since the number of optical components increases as the number of adhesive-bonded points increases, optical components tend to deviate easily during high temperature operation, and deviation in the overall optical system becomes more severe. The large number of components also results in an increase in assembling error during assembling.